Repulsion between electrons with different spins

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Pangol
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A question for specialists...

I have good A Level Physics knowledge, but have never studied Chemistry. I'm trying to learn a bit of A Level Chemistry for the topics that overlap so that I can see a bigger picture.

I'm looking at the basics of shell structure, and in the text book I am using, when introducing spin, it says of two electrons in the same orbital "the opposite spins help to counteract the repulsion between the negative charges of the two electrons". Can anyone tell me if this is actually true, or is a useful simplification for A Level?

Spin itself is not really explained, but I expect it is one of those quantum wierdness things that is hard to be concrete about.
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charco
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(Original post by Pangol)
A question for specialists...

I have good A Level Physics knowledge, but have never studied Chemistry. I'm trying to learn a bit of A Level Chemistry for the topics that overlap so that I can see a bigger picture.

I'm looking at the basics of shell structure, and in the text book I am using, when introducing spin, it says of two electrons in the same orbital "the opposite spins help to counteract the repulsion between the negative charges of the two electrons". Can anyone tell me if this is actually true, or is a useful simplification for A Level?

Spin itself is not really explained, but I expect it is one of those quantum wierdness things that is hard to be concrete about.
It's a good question. Spin isn't usually explained for the simple reason that nobody knows exactly what it is!

We know that the two electrons in a pair are different and that each produces a magnetic field, which cancels out when they are in the same orbital.

We know that a moving electrical charge produces a magnetic field, so it's logical to assume that it is the motion of the electron that causes the field.

Exactly how the electron is moving is not known, so we use the term "spin" to fix its motion and ascribe one of two spin "states" to each electron. We give them quantum values of plus and minus one half, so the difference between them energetically is one quantum of energy.

We also know that atoms "fill up" the orbitals with the lowest energy possible, so it stands to reason that opposite spins is a lower energy state than parallel spins.
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Pangol
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(Original post by charco)
It's a good question. Spin isn't usually explained for the simple reason that nobody knows exactly what it is!

We know that the two electrons in a pair are different and that each produces a magnetic field, which cancels out when they are in the same orbital.

We know that a moving electrical charge produces a magnetic field, so it's logical to assume that it is the motion of the electron that causes the field.

Exactly how the electron is moving is not known, so we use the term "spin" to fix its motion and ascribe one of two spin "states" to each electron. We give them quantum values of plus and minus one half, so the difference between them energetically is one quantum of energy.

We also know that atoms "fill up" the orbitals with the lowest energy possible, so it stands to reason that opposite spins is a lower energy state than parallel spins.
This feels like something that is starting to make sense to me - thank you!

I had suspected that the lack of any attempt to explain spin was becasue it was unexplainable, certainly at this level. I've read many popular books that cover it, and a few university level ones too, but they all seem to work on the principal that you don't ask a question like "So what is spin?", you just get on and use it. Shut up and calculate, as it were.

The magnetic stuff makes a lot of sense, and this is something I understand reasonably well from the physics point of view. But are you saying that the spin up - spin down arrangement counters the usual electrostatic repulsion becasue of magnetic attraction? I would have thought that the electrostatic repulsion would be far stronger. But there again, I have no idea of the parameters involved when it comes to individual electrons.

Very interesting!
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charco
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(Original post by Pangol)
This feels like something that is starting to make sense to me - thank you!

I had suspected that the lack of any attempt to explain spin was becasue it was unexplainable, certainly at this level. I've read many popular books that cover it, and a few university level ones too, but they all seem to work on the principal that you don't ask a question like "So what is spin?", you just get on and use it. Shut up and calculate, as it were.

The magnetic stuff makes a lot of sense, and this is something I understand reasonably well from the physics point of view. But are you saying that the spin up - spin down arrangement counters the usual electrostatic repulsion becasue of magnetic attraction? I would have thought that the electrostatic repulsion would be far stronger. But there again, I have no idea of the parameters involved when it comes to individual electrons.

Very interesting!
No, as I said, with two electrons in an orbital there is no overall magnetic moment. I am just saying that this arrangement appears to be more energetically stable.

Two electrons in one orbital DO repel one another, we know that from the graph of 1st ionisation energy against atomic number. This is particularly true for the inner 's' orbitals, but the effect gets less pronounced as the orbitals get bigger. This is to be expected with electrostatic repulsion. However, the attraction for the nucleus more than overcomes this repulsion.

There is less repulsion within a 'p' orbital than an 's' orbital, usually explained by the more diffuse nature of the 'p' orbitals with respect to 's'.

There have been many models for the nature of atomic orbitals, but current one seems to tick all of the boxes, with the solutions to the Schrodinger equation. I imagine that one day it will be improved upon and further questions will be both answered and raised.
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scimus63
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spin is orbital angular momentum and it is a quantized value, only two states are permitted. There is nothing weird in it. P.W Atkins book - on quantum mechanics gives a good description on what it is and how it is worked out. sorry I had a copy but can't find it or I would have given you the ISBN number - check out amazon
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Pangol
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(Original post by scimus63)
spin is orbital angular momentum and it is a quantized value, only two states are permitted. There is nothing weird in it. P.W Atkins book - on quantum mechanics gives a good description on what it is and how it is worked out. sorry I had a copy but can't find it or I would have given you the ISBN number - check out amazon
I do have access to a copy of that. I would take issue with the statement that there is nothing weird in it!
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scimus63
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found a link- book is a bit pricey - try ebay or uni book store.

https://www.amazon.co.uk/Molecular-Q...0573221&sr=8-3
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scimus63
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I went through this book a few years ago as an amateur with an interest in finding out more on the topic. I used this one and Levines book and I enjoyed them both. The detail in Atkins book was a bit sketchy at times and I found he jumps around too much between topics but for me but I found it helped me grasp the subject matter a little more. Its not perfect by any means but its a start and as I said I enjoyed it. I am not a physicist like you so probably don't have as firm a grasp on the subject but the book was ok for me and it answered more questions for me than it raised.
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